Re: First MMC
dan, Wed Aug 02 2006, 04:45AM

I think before you toss away those salt caps you need to think about getting a better power supply. Like a NST or at least an OBIT. With a rectified power supply (like your flyback) you can have as big of a capacitor as you want. However BPS will suffer if its too big. I wouldn't go above 0.01uF with that flyback just as a ball park figure. Voltage wise shoot for at least 1.5x peak voltage but 2-2.5x is highly recommended.

Re: First MMC
Michael W., Wed Aug 02 2006, 02:01PM

What would my target peak voltage be aprox. ?

Re: First MMC
Terry Fritz, Thu Aug 03 2006, 07:10AM

Hi,

All these calculations are real computer intensive!! Good data in, good data out...

I suggeest you try this thing to find them out:



It is a little scary looking but it works!!!

ScanTesla is also out there, but it is even more geeky



But it all needs lots of "cold hard facts" about your coil system to come up with cold hard answers...

Cheers,

Terry

Re: First MMC
Michael W., Sat Aug 19 2006, 12:43AM

Ok, I've managed to get my hands on a 15KV, 60mA NST. Now I need to build an MMC for it. Any Capacitor recommendations (model number etc) or does anyone have any surplus they'd like to sell? Thanks.

Re: First MMC
Marko, Sat Aug 19 2006, 01:08AM

900W NST is indeed nice, some big sparks I smell

You are going to need a bit moe than 10nF, 30kV MMC for that..

Re: First MMC
Michael W., Sat Aug 19 2006, 01:43AM

I know, I love the NST (I got it free of charge) I could just go an build an MMC on my own but i'm not sure on the best capacitor, capacitor configuration for my money....

Re: First MMC
Coronafix, Sat Aug 19 2006, 03:01AM

I've got an NST of same specs.
The MMC I'm going with is 3 rows of 8
3kv 0.047uf polyproplene
this gives me 17.6nf @ 24kv
this is for a LTR static gap of course.
Don't know about brands and such as they probably wouldn't translate to American.
Total cost is about $100 bucks U.S.
Check out what Chris Hoops has got for his MMC on his 900W coil.

Re: First MMC
Part Scavenger, Sat Aug 19 2006, 03:01AM

The worst thing about MMCs (especially in SGTC's) is that they cost more than salt and aluminum foil. Here's what you need though...

CDE 942 series 2000V capacitors.

For the NST you will need at least 17kV volt rating. Peak Voltage is calculated by V*1.414, if I remember correctly. If possible, use 20kV. WinTesla suggests a tank capacitance of 13.3 nF for your PSU, TeslaCAD suggests 15.9nF. So, this gives you a good range to play with. I'd suggest getting enough caps to try up to 16nF.

The most popular MMC capacitor is the .15uF cap in that series. However, this one would need 10 in series, and a single strand MMC is not supposed to be very stable IIRC. (they tend to overheat, someone back me up on this?) I would find something with a smaller capacitance so that you can parallel and series them. This will make for a more stable MMC.

The best MMC caps are film/foil caps (that's not film or foil, BTW). According to one CDE datasheet, this is because in an overvoltage situation, the capacitor is designed to burn away the foil so it does not short allowing the capacitor to still function nicely "even after repeated overvoltage." There are also some caps form General Electric that EVR likes. I don't recall the part #...

BTW, the reason you don't see many MMC's with flybacks is because the voltage is really high. On average a flyback can put out like 25kV, which means you need about 36kV minimum volt rating. Usually that many caps isn't worth it for the low power.

And... Be careful with your NST! They're fragile. You may already know this, but:

1. Primary strikes are really bad for the transformer (and you BTW). Use a strike rail.
2. To properly set the spark gap, set it so that the NST will just jump it when disconnected from everything. (ie, the spark gap is the only thing connected to the NST) Then, never open the gap farther than that. You can close it all you want, but backing it out too far will overvolt and kill the NST. That's what happened to mine.
3. Safety gaps are handy for catching overvolt on an NST.

Hope that helps.

Re: First MMC
Michael W., Sat Aug 19 2006, 03:52AM

I have the identical Xformer that Chris H. has but he used many polypropolene caps see ( )
I was thinking something more like this...


I just want as cheap as possible. Now looking at the costs of mmc's i'm having second thoughts. I just hate bottles caps as I can't measure thier capacitance, so I can't calculate the amount I need and therefore can't achieve resonance, which gives me 2 inch arcs...

Re: First MMC
Coronafix, Sat Aug 19 2006, 05:48AM

For your NST, resonant charge cap value is 10.6nf. based on 60Hz supply.
Using LTR of 1.4 or 1.5 gives 14.8nf or 15.9nf respectively.
If you're going to spend a bit of money on MMCs then you want to
overvoltage the rating. This is why I say 24Kv.

Re: First MMC
Part Scavenger, Sat Aug 19 2006, 04:12PM



It's a great meter for the price and is suprisingly accurate.

As far as capacitors, you might try making some stack ones with something like glass or transparancies. This sould be cheaper than an MMC, but alot of people have trouble with stack caps.

Re: First MMC
Terry Fritz, Sat Aug 19 2006, 10:33PM

Hi,

For a 15/60 NST and a static spark gap, you want 15.9nF or "close". 10 of the CD 0.15 caps would do fine. There is a chart here to determine such things:



That NST will be a little big for your present coil, so keep the spark gap rather close at first.

Cheers,

Terry

Re: First MMC
Michael W., Sun Aug 20 2006, 03:40AM

Thanks Terry! Now i just have to bite my tounge and press the purchase button. I think digikey has the best deal with the shiiping and all. Now I have to find some 10 M Ohm Resistors. Digikey dosen't sell any rated for 15,000V+...

Re: First MMC
EDY19, Mon Aug 21 2006, 03:19PM

Are the resistors for bleeding the cap bank? If so, put one across each capacitor and it will only see <1500V.

Re: First MMC
Terry Fritz, Mon Aug 21 2006, 09:47PM

Hi,

Mouser.com might have a better deal on the caps depending... I have been using two 4.7M 1/2W carbon resistors across each cap these days.



Also been playing with putting them in a plastic tube "gutter pipe".



The resistors can easily take the voltage. The trick is to put them across "each" cap:



Cheers,

Terry

Re: First MMC
Michael W., Thu Aug 24 2006, 04:45PM

Got my caps today. I decided to buy from digikey as they "seemed" to have the best deal and for only $8, recieved overnight shipping from the states. Although in my haste I think I may have inadvertently ordered 1/4 watt insteat of 1/2 watt 4M Ohm resistors. Is this going to pose a serious problem? Thanks for all your help terry.

Re: First MMC
Terry Fritz, Thu Aug 24 2006, 05:46PM

inadvertently ordered 1/4 watt insteat of 1/2 watt 4M Ohm resistors. Is this going to pose a serious problem?

I would use four of them per cap. I run 1M 1/4W at 300V all the time. But I have never tested 4M or 10M 1/4W resistors for real high voltage use. The 1/2 watts are super well proven now.

Cheers,

Terry

Re: First MMC
Michael W., Thu Aug 24 2006, 08:39PM

Well....I only bought 25 resistors so If they blow out I might just have to order some more....

Re: First MMC
Terry Fritz, Thu Aug 24 2006, 09:59PM

I just tested a 1/4W 10M resistor here.

At 4200VDC it arced over on the outside. Seemed fairly happy at 3000V... So at 2000V peak you should be fine. Seemed the heating at around 3000+V was the biggest problem.

Cheers,

Terry

Re: First MMC
Michael W., Fri Aug 25 2006, 12:02AM

I'll run it low, i'm not feeling to ambitious with my new mmc. I'm going to take all the precautions with it and make a safety gap for my nst etc. (off topic, but not important enough to start another thread) If i'm using a variac to control the nst should my safety gap be the distance of the arc at full power? (i'm guessing so)

Re: First MMC
Avalanche, Fri Aug 25 2006, 02:28AM


woh, are these actual high voltage rated resistors, or just any old standard resistor? Only reason I ask is I have an almost complete spark gap coil sitting here (super secret project!) but no resistors across the MMC caps. My local stores are out of the HV rated ones, so I daren't fire the thing up just yet in case I kill the MMC

Re: First MMC
Michael W., Fri Aug 25 2006, 03:03AM

They are just regular carbon film resistors, I just bought the wrong ones....Just out of intrest how could a lack of resistors kill the mmc?

Re: First MMC
Self Defenestrate, Fri Aug 25 2006, 03:53AM

The resistors balance out the voltages across the caps, and help bleed it after its turned off.